Linear synchronous motors (hereinafter "LSM") are well known and have been successfully used for moving vehicles along an elongated path. Laithwaite, Proceedings of the IEEE, "Linear-Motion Electrical Machines," Vol. 58, No. 4, April 1970. In order for a LSM to operate properly, the secondaries, which move along the fixed primary, have salient spaced apart magnetic poles whose fields synchronize with, or "lock" onto, the traveling electromagnetic (hereinafter "EM") wave developed in the primary. Therefore, when the field of a secondary is synchronized with the EM wave, such secondary will move along the primary at the velocity of the EM wave.
The EM wave has characteristics of velocity, polarity and pole pitch. The EM wave is developed in the primary by powering spaced coils of the primary by a multiple phase alternating current (hereinafter "AC") or power waveform. The velocity of any specific secondary propelled along the primary by an EM wave developed in the primary is determined by the following expression: EQU U=2.lambda.f
where, ".lambda.", in accordance with electric motor manufacture nomenclature, is the pole pitch between two adjacent poles of the EM wave, and "f" is the frequency of the AC waveform powering the EM wave. In the above expression, .lambda. equals the distance per 1/2 the cycle of the EM wave. So, the EM wave travels one pole pitch for every 1/2 of an AC cycle of the power waveform. Since the pole pitch is fixed in the winding pattern in the primary, the pole pitch and pole spacing of the secondaries are matched to that of the fixed primary for any given LSM system. Therefore, by varying the frequency of the power waveform, the velocity of the EM wave is varied and, likewise, so is the velocity of the secondaries "locked" onto the EM wave.
Loss of synchronism between a particular secondary and the traveling EM wave is a problem in LSM systems. This loss of synchronism prevents accurate speed and position control of the secondary, and in the extreme case can cause the secondary to stop. Loss of synchronism can occur because of excessive loading on the secondary due to external loads and forces developed in accelerating (either positively or negatively) the secondary with the EM wave. Loss of synchronism can also occur when independently controlled adjacent zones along the primary are not properly coordinated so that the EM waveform developed in each zone is matched at the interface between such adjacent zones.
The effect of loss of synchronism varies with the type of LSM system. In an LSM system in which the vehicle attached to a secondary has a high mass and is traveling at high speed, the inertia is large compared to the propulsion force. Loss of synchronism in this type of system is not severe and can be corrected during operation by, for example, feedback circuitry. This type of LSM system is more forgiving in that if loss of synchronism occurs, the secondary will continue along the path because of the large amount of inertia associated with it and there is ample amount of time for the EM wave to be adjusted by the feedback circuitry to match the new velocity of the secondary and after such adjustment the secondary will again "lock" onto the EM wave. However, accurate knowledge of the position of the secondary propelled by the EM wave may be lost.
In LSM systems in which the vehicle attached to the secondary has a low mass and travels at a low speed, the inertia is low compared to the propulsion force. In such systems, loss of synchronism can be severe and cause the vehicle attached to the secondary to oscillate or come to a stop. This type of system is unforgiving and adjustment of the EM wave through feedback circuitry would not normally be successful because the feedback adjustment procedure would not have sufficient time to adjust the EM wave before the secondary will have come to a stop.
These two systems are at the extremes of the effect on LSM systems of the loss of synchronism between a secondary and the EM wave. All other systems, such as high inertia-high force or low inertia-low force systems, when experiencing loss of synchronism, will react somewhere in between the two. So, it is very critical in all LSM systems, which are very unforgiving, to ensure that loss of synchronism does not occur because it may result in total shut down of the system.
In a LSM system which has multiple zones in which each specific zone is independently powered to develop an EM wave, there is a problem in maintaining precise control in powering the primary to match the power waveform and, therefore, the EM wave at the interface between adjacent zones. As a secondary crosses the interface, or is "handed off", between two independently controlled zones, the EM wave developed for each zone for a period of time propels this same secondary. The magnetic pole velocity and polarity of the traveling EM wave developed independently in the adjacent zones must match each other during "hand-off" or synchronism between the EM wave and the secondary will be lost. The mismatch will exist if either the frequency or phase of the two power waveforms, which develop the EM wave independently in each adjacent zone, are not the same at their interface.
In situations when the independently controlled adjacent zones propel the secondary at a constant velocity or increase or decrease the velocity of the secondary, matching of the power waveforms of adjacent zones during hand off at the interface is a problem.
A prior art method used for effecting the "hand-off" of a secondary from one zone to another, is described in U.S. Pat. No. 3,803,466 (hereinafter "the '466 patent"). In the system of the '466 patent, transition from one zone to another in which acceleration of the secondary is contemplated, is achieved in the following manner. As rotors (secondaries) approach the end of, for example, a constant speed zone, the frequency and phase of the power waveform produced in the stator (primary) in the subsequent, adjacent acceleration zone is adjusted to match that of the constant speed zone via feedback circuitry. As the rotors transition from the constant speed zone to the acceleration zone, the powering waveform has the same phase and constant frequency in both zones. Once the rotor has completely moved into the acceleration zone, a switch is tripped by the rotor that causes the frequency of the power waveform in the acceleration zone to increase. To transition to the next constant speed zone having a higher velocity, or to a second acceleration zone, it is necessary to achieve a constant frequency and phase match at "hand-off" between the two adjacent zones, as previously described. This type of system requires time to achieve synchronism, and substantial amounts of feedback circuitry and sensing to ensure the frequency and phase are properly matched when the rotor is "handed-off" from the constant speed zone to the acceleration zone, or from the acceleration zone to a constant high speed zone, or from one acceleration zone to another.
The present invention provides a system and method that overcomes the limitations and disadvantages of prior art systems as will be described.